Week 2: Pepcin, Mucin and Intrinsic Factor

Question 1

Where are Chief Cells located?

Answer 1

Cryptic pits in the stomach infolds

Question 2

Chief Cells secrete?

Answer 2

Pepsinogen

Question 3

What is pepsinogen?

Answer 3

a precursor to protease which is inactive ‘ogen’ suffix is indicative of inactivity

Question 4

What types of proteases are there?

Answer 4

• Endoproteases
• Exoproteases

Question 5

What is an endoprotease?

Answer 5

protease that digests from the middle or not from the ends of proteins

Question 6

What is an exoprotease?

Answer 6

Digests proteins from the terminal ends

Question 7

what does the suffix ‘ogen’ mean

Answer 7

inactive

Question 8

Why is it important that pepsinogen is inactive?

Answer 8

because you don’t want proteases digesting intracellular proteins

Question 9

What is autoproteolysis?

Answer 9

when an inactive protease is able to cleave itself when a certain condition is met (eg pepsinogen to low pH) to create a conformational change which becomes the active protease

Question 10

What kind of protease is pepsinogen?

Answer 10

endoprotease

Question 11

Optimal pH for pepsinogen?

Answer 11

pH of 2-3

Question 12

What is the active form of pepsinogen?

Answer 12

Pepsin

Question 13

What happens when pepsin exits the stomach into the duodenum?

Answer 13

Pepsin activity is reduced or could be 0

Question 14

what is the typical pH of the duodenum?

Answer 14

pH ~8.0

Question 15

What are goblet cells?

Answer 15

filled with translucent material (mucopolysaccharide)

Question 16

What is the typical structure of a mucopolysaccharide?

Answer 16

typically have very short protein domains and very long carbohydrate domains

This structure binds a lot of water by the sugars and interacts with a lot of H-bonds which makes it mucus-like and viscous

Question 17

Why is mucin important?

Answer 17

mucin gelatinous layer sits on top of cell surfaces along the lumen of the GI tract

The gelatinous layer protects the cells from active pepsin preventing pepsin from digesting membrane proteins

Goblet cells also secrete HCO3- which is basic

we have low pH in the lumen so the HCO3- binds to the protons to neutralize, however the HCO3- sticks in the mucus which protects the underlying cells from the cells surface preventing disruption of luminal membrane proteins

Question 18

Where does intrinsic factor come from?

Answer 18

Parietal cells

Question 19

Intrinsic factor function

Answer 19

Vitamin B12

Question 20

Why is vitamin B12 critical?

Answer 20

• Because it is important for 1carbon metabolism
• maintaining RBCs in an antioxidized state

Question 21

important chemical properties of vitamin B12

Answer 21

very hydrophobic so it binds to proteins very tightly

Question 22

How is Vitamin B12 absorbed

Answer 22

Vitamin B12 binds to any protein

Pepsin hydrolyzes that protein exposing Vitamin B12 which then binds to intrinsic factor which acts as a carrier

Intrinsic factor+Vitamin B12 is then travels through the intestines to the ileum and there are receptors in the ileum which bind the Intrinsic factor+B12 complex

Intrinisic factor is degraded and releases the B12

B12 enters the blood stream by secretion from the basal surface of the epithelium and complexes with transcobalamin

Transcobalamin is a plasma protein which is specific to bind vitamin B12

The Transcobalamin+B12 complex can then enter cells as needed

Question 23

What is transcobalamin?

Answer 23

• Transcobalamin is a plasma protein which is specific to bind vitamin B12 and acts as a carrier
• The Transcobalamin+B12 complex can then enter cells as needed